Kathy O'Brien was a graduate student that approaches Sheldon and was very impressed by the paper that Sheldon and Ramona Nowitzki worked on and published. She also wanted to discuss the paper with him and offered to pick up his weekly pizza. Sheldon does not realize he is getting picked-up for the second time in the episode. He does enjoy getting free food and is impressed by the intelligence shown by these two graduate students since he expected none from any of the new students.
Kathy is played by Emil Happe.